The interior walls of buildings, for example houses, offices, restaurants, retail stores, hospitals and the like typically include a frame lined with plasterboard panels. The frame of the wall normally includes a series of upright beams, commonly referred to as studs, to which the plasterboard panels are mounted. The panels are mounted to the studs such that the edges of adjoining panels abut one another. The edges are then covered with wet plaster and subsequently sanded when the plaster dries to provide a continuous wall surface. The wall surface created by the plasterboard panels is also usually painted to provide an aesthetically pleasing appearance.
In general, hard, solid materials, for example plasterboard panels, reflect sound better than softer air permeable materials. In this respect, sound waves incident upon an interior wall lined with plasterboard tend to be reflected well. The reflected sound waves can also undergo reflection by bouncing off other walls and surfaces, even after the source ceases emitting sound. This phenomenon is known as reverberation and the time it takes for reverberant sound energy to dissipate by 60 dB is known as the reverberation time. The reverberation time in an enclosure, for example a room, can make a significant impact upon the intelligibility of speech. In this respect if the reverberation time is too long speech can be difficult to interpret as the reverberant sound in the room acts as background noise.
Ideally, the issue of reverberation is considered and addressed at the design stage of a building. However, in some instances, reverberation problems may not become apparent until construction of a building is completed. In both cases there are various options available to address reverberation issues. These typically include the use of perforated acoustic tiles, carpet, curtains, fabric wall linings and other soft materials. Unfortunately, many of these options are not able to adequately blend with the desired aesthetic appearance.
The acoustic panel disclosed in International Publication No. WO 2009/023900, (herein after referred to as “the Bellmax panel”), the contents of which are herein incorporated by reference, sought to address the issue of aesthetic appearance by providing a sound absorbing acoustic panel which mimicked the look and feel of a conventional plasterboard panel, could be painted like conventional plasterboard yet remain sound absorbing, and be installed using the same installation method as conventional plasterboard. Although the Bellmax panel had the desired aesthetic appearance, its ability to absorb sound wave energy once painted was found to be limited to specific frequencies.
FIG. 1 of the accompanying drawings provides a graph showing the absorption coefficient across a range of frequencies for a painted sample Bellmax panel of the prior art. The sample Bellmax panel consisted of a membrane layer made of paper, and an underlying sound absorbing layer made of fibrous polyester having a surface density of approximately 1800 g/m2 (without apertures). The sound absorbing layer had a plurality of 15 mm apertures extending therethrough which provided the layer with 33% open area. The sample was mounted to a frame structure having wall type studs with sound absorbing material having a surface density of approximately 800 g/m2 being located behind the sample in a wall type cavity having a depth of 25 mm.
The graph in FIG. 1 demonstrates that the sample Bellmax panel has two prominent absorption peaks 2, 4 at approximately 300 Hz and 1700 Hz with virtually no absorption being provided at other frequencies. The absorption peak 2 at 300 Hz is due to the sample Bellmax panel, in combination with the enclosed air volume in the wall type cavity behind the sample, acting as a panel absorber. In this respect, a panel absorber is a form of resonant oscillating mass-spring system whereby the panel is able to resonate in response to sound waves incident on the panel with dampening being provided by the enclosed air volume. The absorption peak 4 at 1700 Hz is due to the portions of the membrane layer which overlie the 15 mm apertures acting as diaphragms. These diaphragms vibrate at maximum amplitude when imparted with sound waves of a frequency corresponding to their resonant frequency, thereby reducing the sound waves energy. Apertures of different and increased size could be used to broaden the range of frequencies absorbed below 1700 Hz. However, suitable absorption at frequencies between approximately 350 Hz and 800 Hz and lower than 350 Hz would require much larger apertures in the Bellmax panel and/or a much greater depth in the wall cavity behind the sample. A problem with increasing the size of the apertures is that it becomes increasing difficult for the portions of the membrane layer which overlie the apertures to retain a surface finish which can mimic that of a conventional plasterboard panel, particularly after the membrane layer is painted. In addition, the depth of the enclosure behind the panel can not be readily increased beyond the fixed depth of the wall cavities of the building.
In view of the above, it would be desirable to provide an acoustic panel having a variety of applications which is able to absorb sound wave energy across a broad range of frequencies. It would also be desirable to provide an acoustic panel which when used as an interior wall lining, is able to mimic the look and feel of a conventional plasterboard panel when painted. Such a panel could be used to reduce reverberation within the interior of buildings. It would also be desirable to provide an acoustic panel which could be used in automotive applications, for example on the firewall of an engine bay or as an underbody lining, to reduce sound wave energy and heat transfer to the passenger compartment of a motor vehicle. It would further be desirable to provide an acoustic panel which could be used for outdoor applications such as a roadside absorptive barrier.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein.